High accuracy interferometers have been widely used for surface measurement not only in optical manufacturing but also in new fields such as optical disk surface measurement and semiconductor crystal plane measurement. More and more emphasis has been placed on detection accuracy of peak-to-valley (“PV”) values in the sub-nanometer range. FIG. 8 schematically shows a system configured according to a conventional six-step absolute flatness detection technique. As shown in FIG. 8, the system comprises a laser light source 1a, a collimation optical system 3a and 4a, a beam splitter 2a, a front surface 5a of a first test flat, a piezoelectric transducer (“PZT”) phase shifter 6, a front surface 7a of a second test flat, a convergence optical system 8a, a charge-coupled device (“CCD”) 9a, and a computer 10a. 
FIG. 9 schematically shows a measurement process of the system in FIG. 8. The measuring process includes:                (1) measuring an optical path difference between the front surface of the first test flat and the front surface of the second test flat;        (2) rotating the first test flat by 180° from its original position, and measuring an optical path difference between the front surface of the first test flat and the front surface of the second test flat;        (3) rotating the first test flat by 90° from its original position, and measuring an optical path difference between the front surface of the first test flat and the front surface of the second test flat;        (4) rotating the first test flat by 45° from its original position, and measuring an optical path difference between the front surface of the first test flat and the front surface of the second test flat;        (5) replacing the second test flat with a third test flat, and measuring an optical path difference between the front surface of the first test flat and the front surface of the third test flat;        (6) replacing the first test flat with the second test flat, and measuring an optical path difference between the front surface of the second test flat and the front surface of the third test flat.The first test flat, the second test flat, and the third test flat are calculated based on the measurement results. The foregoing six-dimensional angular rotation platform typically has an accuracy of about 10 micro-radians to about 1 milli-radian. Such accuracy levels may not satisfy the ever increasing accuracy requirements in today's industry. Accordingly, there is a need for apparatus and methods that can improve the measurement accuracies of conventional surface measurement platforms.        